1. Field of the Invention
The present invention relates to a scanning optical system that scans a surface by use of multiple light beams, and more particularly, to a passive thermal compensation mechanism for maintaining the apparent position of an aperture in the multiple beam scanning optical system though a change in temperature.
2. Description of the Related Art
Multiple beam scanning optical systems in which multiple light sources are used, such as those disclosed in U.S. Pat. Nos. 5,300,956 and 5,471,236, for example, are conventionally known. These multiple beam scanning optical systems have an aperture to regulate the diameters of the multiple beams.
However, in conventional scanning optical systems, when there is a change in the environment, or more particularly when the ambient temperature changes, the oscillating wavelengths of the laser diodes fluctuate, and at the same time changes in the configuration and refractive index of the condenser lens occur. During the time the focal length of the condenser lens fluctuates, because the construction is not such that the aperture is moved in response to the temperature change, a portion of the light beams that should pass through the aperture is blocked by the aperture and the intensities and intensity distributions of the beams on the scanned surface after passing through the aperture change. This is because the aperture is no longer at the image side focal point of the condenser lens, i.e., a position at which the laser beams projected through the condenser lens cross the optical axis.
Thermal compensation in camera systems is also known, such as disclosed in U.S. Pat. Nos. 1,325,936 and 2,533,478. In these patents, the thermal expansion of cantilevered lens supports adjust the position of lens elements to maintain focus through temperature changes. A thermalized optical imaging systems are also known from, for example, U.S. Pat. No. 5,283,695, which also discloses a cantilevered lens support. Being drawn to camera systems and single beam scanning systems, however, these patents do not address the problems caused by a shift in the apparent position of an aperture in a multiple beam scanning optical system. In a multiple beam scanning optical systems, as explained above, if the aperture is moved in response to the temperature change, a portion of one or both of the light beams that should pass through the aperture is blocked by the aperture and the intensities and intensity distributions of the beams on the scanned surface after passing through the aperture. This problem would not be relevant to a camera system or a single beams optical system, which are more tolerant of intensity changes and would not suffer from changes in the intensities and intensity distributions of beams on the scanned surface after passing through the aperture as multiple beam scanning optical systems do.